They are not alternatives - they are different things, and as far as I know, they can be done together.
Retzius sparing is a different way of dissecting out the prostate which avoids damage to an anatomical area called the Cave of Retzius. This tends to give a faster recovery of urinary continence after the operation, but 6-12 months later, Retropubic (standard) RALP patients have caught up. Another significant factor is it tends to be only the most experienced surgeons who do Retzius sparing RALP.
Neurosafe (also called Frozen Sections) is a technique for avoiding positive margins, and may improve the chances of nerve sparing without increasing the risk of positive margins. Positive margins are where not all the cancer was cut out, and without Neurosafe, it's not normally discovered until after the operation and it's too late to fix it in the operation, and radiotherapy is likely to be required. Neurosafe enables the positive margin to be seen while you're still in theatre, enabling it to be cut away.
There's an alternative procedure using a Histolog Scanner which does the same job as Neurosafe, which some hospitals are just starting to use. (It doesn't require the Histopathologist to be in/near the operating theatre, as it works across the Internet.)
As to which to chose if you can only have one, I think this depends if faster return of urinary continence (Retzius sparing) is more important to you than reducing the risk of needing salvage radiotherapy (Neurosafe). There are no guarantees either way though.